1. Field of the Invention
The present invention relates to a plate type evaporator wherein a liquid to be evaporated is evaporated by a heating medium.
2. Description of the Prior Art
In an electric generating plant for generating power by circulating a working medium such as water or an organic material, a chemical plant for refining materials, a refrigerating machine and other equipment, an evaporator is used without exception. In many cases, the heat transmitting section of an evaporator used in such plant is in the form of a system of round pipes having smooth inner and outer surfaces or a system of horizontally or vertically disposed round finned-pipes with the fins attached to the outer side of the pipes for improving the rate of heat exchange.
However, in an evaporator having a system of round pipes as described above, it is extremely difficult to improve its performance for the following reasons.
(A). In cases where said round pipes are horizontally or vertically disposed and a heating medium is passed outside the pipes while a liquid to be evaporated is passed inside the pipes for heat exchange, the pipes have to be more densely arranged in order to lower the cost. As a result, the pressure loss of the heating medium is increased and the power required is also increased. Further, since the liquid to be evaporated is passed inside the pipes, unstability of flow due to the so-called two-phase flow phenomenon is caused and depending upon the amount of the liquid to be evaporated there will be a part which does not at all contribute to heat transmission.
(B). In cases where a heating medium is passed inside the pipes while a liquid to be evaporated is passed outside the pipes for heat exchange, about half of the external area of the pipes does not contribute to heat transmission.
(C). In addition to the disadvantages described above, such pipe type evaporator requires much time and labor in drilling end plates for attaching the heat transmitting pipes and assembling the entire evaporator. Moreover, the heat transmitting pipes themselves are very expensive.
Further, in some chemical plants, a plate type evaporator which can be readily fabricated is used. However, in a known plate type evaporator, sets of plates each consisting of two are put together so that a heating medium and a liquid to be evaporated flow alternately and since the liquid is supplied in one direction it is difficult to supply it uniformly over the wide and long heat transmitting surfaces. As a result, scale scorches and sticks to the surfaces of the plates on the evaporating side, greately lowering the heat transmission effect and decreasing the available running time. Reversely, if a liquid to be evaporated is supplied over the entire heat transmitting surfaces, the thickness of the liquid film gradually increases in a longitudinal direction, greately decreasing the heat transmission effect.